What should a voltmeter read between the negative cable and the ground post on the battery on a 1999 mountaineer? I am reading 12.9. I was told it should be 10 0r 11. If so, is my battery being drained by a faulty system? I have pulled every fuse and it still reads a constant 12.9.

You don’t use a voltage meter for that. You need a current meter. Do you have a multimeter with a current setting? You need to put it on the 10 or 20 amp range and then do your test. You will be looking for a reading of less than 50 mA (0.05 A). It may take up to an hour for it to settle down to that. It sounds like you need to get some in-person help from someone with more electrical knowledge than you.

What should a voltmeter read between the negative cable and the ground post on the battery ,

Zero. There should not be a difference unless you remove the cable. Then it might be about 12 volts, but that would depend on the resistance of your meter. Volts in that case is meaningless. I suspect you want to measure current (amps) not voltage For that your volt meter likely will not be able to measure it. You would need an amp meter.

Give is a little more to go on. What exactly is happening that makes you suspect a problem.

The way you stated your method of testing is a little hard to understand. I assume you meant to say that you did the voltage test with the negative cable disconnected from the battery. Like the others said, this isn’t the best way to check for a current draw. Use the current function of the meter if you have it. As Tardis mentioned, you may need to wait a while for the systems to do into the sleep mode after power is connected back up. The current draw will seem high until the systems go to sleep.

You can monitor the voltage across the battery leads with a voltmeter but it will take time to see if there is a problem. If there is a problem the voltage will slowly drop down. This could take a few hours to see a few tenths of a volt change.